PSEN1 and early-onset autosomal dominant Alzheimer disease: Numerous genes involved in the pathogenesis of Alzheimer disease have been described so far, including, but not limited to, mutations in amyloid precursor protein (APP), presenilin 1 and 2 (PSEN1 and PSEN2), and carrying the E4 allele of Apolipoprotein (ApoE) genes.